The present invention relates to an open drain output circuit.
An I2C (Inter-Integrated Circuit) bus interface includes a control macro and an I/O circuit. An open drain output circuit is used as the I/O circuit. The I2C bus has a predetermined standard related to the delay characteristic of the output signal. Therefore, a stable I2C bus interface must be configured with the output property of the open drain output circuit conforming to the standard.
FIG. 1 shows one example of the I2C bus interface. The I2C bus interface includes a control macro 1 and I/O circuits 2a and 2b. In accordance with DATA (SDA) and CLOCK (SCL) output from the control macro 1, the transmission of DATA is performed.
FIG. 2 shows one example of an input/output signal transition in the I/O circuits 2a and 2b. Input signals DATA IN and CLOCK IN are respectively provided to the I/O circuits 2a and 2b. Then, the I/O circuits 2a and 2b respectively generate output signals DATA OUT and CLOCK OUT based on the predetermined delay characteristic. In the example of FIG. 2, the input signals DATA IN and CLOCK IN of the I/O circuits 2a and 2b have opposite phases. The output signals DATA OUT and CLOCK OUT also have opposite phases.
The input signals DATA IN and CLOCK IN are provided to the I/O circuits 2a and 2b as synchronous signals. The output signals DATA OUT and CLOCK OUT are set so that the output signal DATA OUT rises to an H level after the output signal CLOCK OUT falls to an L level due to the delay characteristic of the I/O circuits 2a and 2b based on the standard of the I2C bus interface.
A predetermined gradient is set for the falling characteristic of the output signal CLOCK OUT and for the rising characteristic of the output signal DATA OUT.
FIG. 3 shows a conventional open drain output circuit 100 used as an output circuit of the I/O circuits 2a and 2b. An input signal IN is provided via a buffer circuit 3 to the gates of the transistors Tr1 and Tr2, which configure an inverter circuit 4. The transistor Tr1 has a source connected to a power supply Vcc by way of a resistor R1. The transistor Tr2 has a source connected to ground GND.
The output node N1 of the inverter circuit 4 is connected to the gate of an output transistor Tr3 (N-channel MOS transistor). The output transistor Tr3 has a drain, connected to an output terminal To, and a source, connected to the ground GND. A capacitor C1 is connected between the drain and gate of the output transistor Tr3.
The output terminal To is connected to a bus. A terminal resistor R2 is connected between the bus and the power supply Vcc. A bus capacitor C2 (sum of wiring capacitance and input capacitance of other LSIs etc. mounted on a circuit board) is connected between the bus and the ground GND.
In the open drain output circuit 100, the node N1 falls from an H level to an L level (first operation) when the input signal IN rises from an L level to an H level, as shown in FIG. 4. The output transistor Tr3 is then turned OFF and the output terminal To is set to a high impedance state. Thus, an output signal Vout generated at the output terminal To gradually rises from an L level to an H level in accordance with the time constant determined by the terminal resistor R2 and the bus capacitor C2.
Next, the potential at the node N1 gradually rises from an L level to an H level (second operation) when the input signal IN falls from an H level to an L level. This is because the drain current of the transistor Tr1 is restricted by the resistor R1.
When the potential at the node N1 reaches a threshold value of the output transistor Tr3, the output transistor Tr3 is turned ON, the drain current of the output transistor Tr3 gradually increases as the potential at the node N1 rises, and the output signal Vout gradually decreases. The gradient of the output signal Vout is set so as to comply with the standard of the I2C bus interface through adjustment of the resistance of the resistor R1 and the capacitance of the capacitor C1.
Japanese Laid-Open Patent Publication No. 11-346147 discloses a through rate output circuit for shortening the delay time of the output signal with respect to the input signal while having a through rate function for making the rise and fall of the output signal gradual.
Japanese Laid-Open Patent Publication No. 7-30399 discloses an open drain output circuit in which the gradient for the fall of the output signal is adjustable.
Japanese Laid-Open Patent Publication No. 11-274909 discloses an open drain output circuit in which the gradients of the fall and the rise of the output signal are adjustable.
Japanese Laid-Open Patent Publication No. 2004-266494 discloses a configuration for reducing the power consumption of the open drain output circuit.